1. Field of the Invention
The present invention relates to an optical element that has light-diffracting properties, to an optical system having such an optical element, and to a method for manufacturing such an optical element. More particularly, the present invention relates to all optical element having a blaze-shaped diffraction grating (i.e., a diffraction grating that has a serrated cross section composed of a series of rectangular triangles), to an optical system having such an optical element, and to a method for manufacturing such an optical element.
2. Description of the Prior Art
In recent years, various techniques have been proposed for correcting chromatic aberration with as few lens elements as possible through the light-dispersing action of a lens surface that has light-diffracting properties.
For example, the taking lens of a single-lens camera employs resin-molded lens elements that help minimize chromatic aberration, and, as one preferable example of such resin-molded lenses, Japanese Patent Publication No. H8-508116 proposes a diffractive optical element that adopts a light-diffracting construction to correct chromatic aberration.
This prior-art lens is a refraction/diffraction-hybrid-type achromatic lens (i.e., a lens that is made achromatic through the action of both refraction and diffraction) made of transparent resin, and is suitable for a single-lens camera that operates in the visible-light region. More specifically, at least one of the optically relevant surfaces of this lens is not only curved to form a refractive region having a refractive power but also provided with a diffractive region having a diffractive power to reduce the chromatic aberration caused by the refractive region.
In a diffractive optical element, as described above, that acts as a lens through the action of a diffraction grating, it is essential that the element have a stronger diffractive power in its peripheral region, than in its central region such regions being with respect to and around the optical axis of the optical element. To make the diffractive power in the peripheral region stronger, the pitch of the grating in that region needs to be made finer. Inconveniently, however, a fine-pitched diffraction grating is difficult to produce. European Patent No. 0412751 proposes a diffractive optical element in which a relatively strong diffractive power is achieved not by making the grating pitch finer but by designing a grating of a higher order. In this diffractive optical element, to obtain a grating of a higher order, the diffraction grating is designed to have larger grating heights in its peripheral region than in its central region, such regions being with respect to and around the optical axis of the optical element.
Inconveniently, however, the diffraction grating proposed in this European Patent No. 0412751 exhibits poor diffraction efficiency in its peripheral region and causes eclipse in that region.
To cope with these problems, Japanese Published Patent Application No. H7-50206 proposes a grating that is so shaped as to selectively produce diffracted light of the orders 0 (zero) and .+-.1 (the shape is defined in terms of the bi-level heights of the grating, the inclination of the surfaces forming the convex regions of the grating, and the difference in the refractive index between the convex and concave regions); on the other hand, Japanese Laid-open Patent Application No. H5-150108 proposes a grating that is so shaped as to offer desired phase-shift characteristics. However, these gratings are not so shaped as to act as lenses, and therefore the use of such grating shapes in a diffractive optical element does not help prevent reduction in the diffraction efficiency or eclipse as mentioned above.
When a diffractive optical element is manufactured through resin molding by the use of a mold, a blaze-shaped pattern formed on the surface of the mold is transferred onto resin material. At that time, the finer the pitch of the diffraction grating, the less negligible the transfer errors that occur during the transferring process, particularly in the peripheral region. Inconveniently, this adds to the difficulty of producing a fine-pitched diffraction grating.